False memories for true and false vaccination information form in line with pre‐existing vaccine opinions

Abstract Misinformation continually threatens efforts to control the COVID‐19 pandemic, with vaccine misinformation now a key concern. False memories for misinformation can influence behavioural intentions, yet little is known about the factors affecting (false) memories for vaccine‐related news items. Across two experiments (total n = 1481), this paper explores the effects of pre‐existing vaccine opinions on reported memories for true and false news items. In Study 1, participants (n = 817) were exposed to fabricated pro‐ or anti‐vaccine news items, and then asked if they have a memory of this news event having occurred. In Study 2, participants (n = 646) viewed true pro‐ or anti‐vaccine news items. News items were more likely to be remembered when they aligned with participants' pre‐existing vaccine beliefs, with stronger effects for pro‐vaccine information. We conclude by encouraging researchers to consider the role of attitudinal bias when developing interventions to reduce susceptibility to misinformation.


| INTRODUCTION
Misleading and biased information has increased in both accessibility and quantity since the introduction of the internet (Lazer et al., 2018). Throughout the COVID-19 pandemic, online misinformation has proliferated (Brennen et al., 2020;Kouzy et al., 2020), leading the WHO director-general to declare an 'infodemic'. As researchers focus on developing the best interventions for the spread and impact of misinformation surrounding COVID-19, it remains vital to continue to examine the mechanisms underlying why individuals believe, spread, and act on misinformation. Recent research has demonstrated that exposure to 'fake news' can result in false memories for the events depicted (Frenda et al., 2013;Murphy et al., 2019), and that people who form such false memories may be more likely to act on the misinformation . The unprecedented rise in online COVID-19 fake vaccine news therefore adds urgency to understanding the factors influencing false memory development.
Recent research indicates that susceptibility to false memories is influenced by a range of individual differences including personality traits, intelligence, and age Lee, 2004;Roediger III & Geraci, 2007;. False memories are considerably more likely to form when the news item or event aligns with participants' ideological or political viewpoints (Frenda et al., 2013;Murphy et al., 2019Murphy et al., , 2021. Levels of interest in or expertise for a topic have also been linked with explanation for the finding that false memories tend to form in line with individuals' knowledge, beliefs and interests: prior experiences and knowledge may influence the perceived plausibility of an event, which in turn influences belief that the event actually happened (Mazzoni et al., 2001;Scoboria et al., 2004Scoboria et al., , 2007. Fuzzy trace theory (Reyna et al., 2016) provides an alternative theoretical framework, in which false memories arise as a result of parallel formation of 'verbatim' and 'gist' memory traces for experienced events. While verbatim traces record specific surface details of a given event (e.g. remembering that a pedestrian was struck by a 2018 Toyota Corolla), gist traces provide a boiled-down representation of the essential characteristics (e.g. remembering a person being hit by a car). According to this model, false memories arise due to overlap in gist representations, such as when a witness confuses their memory of a witnessed traffic accident with another they saw on television; both events contain the same essential features-the gist-but differ in their specific (verbatim) features (Brainerd & Reyna, 2005).
Research suggests that people tend to rely on gist representations when making decisions (Reyna & Lloyd, 2006). For example, when faced with the decision to get vaccinated against a disease, an individual will extract the essential features of vaccine-relevant information they have encountered in the past (i.e. the gist) and use this information in making their decision. Of course, the determination of what features are essential will be determined both by the quality of information available to the individual, and their own background knowledge (Reyna, 2012). Thus, someone who has encountered a lot of anti-vaccination misinformation and who does not have a strong scientific background may erroneously derive the conclusion that vaccines are unsafe. Upon encountering a new piece of information (e.g. an anti-vax fake news story) that overlaps substantially with this gist, the individual may falsely remember having encountered the information before.
The tendency to remember items which align with our prior knowledge and experience is not unique to misinformation. Bartlett's (1932) seminal work on schemas demonstrated that memory tends to be stronger for information which aligns with prior knowledge than information that does not. Such findings have been supported by numerous studies (e.g., Anderson, 1981;Tse et al., 2007;van Kesteren et al., 2012). In contrast, evidence that memory may sometimes be enhanced for unexpected or schema-inconsistent information has also been reported (e.g., Greve et al., 2017;Mäntylä & Bäckman, 1992;Tulving & Kroll, 1995). Greve et al. (2019) explored this apparent contradiction across four experiments with simplified stimuli. They reported a U-shaped pattern in memory performance, whereby schema-consistent and schema-inconsistent information tends to be remembered at higher rates than information that is not relevant to the schema. It remains to be seen whether a similar pattern will be apparent in the reporting of memories for more complex information, such as true and false news items. Moreover, previous work on false memory formation following exposure to ideologically congruent misinformation has focussed on political news. There is as yet no evidence about the effects of pre-existing opinions on false memories for health-related information.

| The present paper
The COVID-19 pandemic and rollout of COVID-19 vaccines is an ideal context for exploring the reporting of memories. Understanding the factors influencing how individuals 'remember' both fabricated and accurate news may provide a better understanding of the influence of COVID-19 vaccine information and misinformation. Based on previous literature, we expect that memories for both true and false information about the COVID-19 vaccines will be more likely to form in line with participants' pre-existing views about vaccination. Evidence suggests that the pattern of vaccine hesitancy and anti-vaccination attitudes may be different for the COVID-19 vaccine than vaccines in general (Dror et al., 2020;Fridman et al., 2021). We therefore evaluated effects of pre-existing opinions on vaccination in general, and COVID-19 vaccination in particular. All measures, manipulations and exclusions are reported. The present studies investigated susceptibility to false memories for fabricated news headlines that were either in favour of or opposed to vaccination against COVID-19 (Experiment 1) and reporting of memories for accurate COVID-19 vaccine news headlines (Experiment 2). Only participants who had yet to be vaccinated were included. The hypotheses for these studies are as follows: • Experiment 1: Participants with more negative attitudes towards vaccines will be more likely to report a memory for fake news stories that are negative about COVID vaccines, while those with more positive attitudes towards vaccines will be more likely to report a memory for fake news stories that are positive about COVID vaccines.
• Experiment 2: Participants with more negative attitudes towards vaccines will be more likely to report a memory for accurate news stories that are negative about COVID vaccines, while those with more positive attitudes towards vaccines will be more likely to report a memory for accurate news stories that are positive about COVID vaccines.
All materials and data for both experiments are available at https://osf.io/jw23x/.

| Pre-registration
These data were collected as part of a larger study examining effects of misinformation on vaccine intentions .
The present paper addressed distinct research questions, and was thus separately preregistered at https://aspredicted.org/55a5y.pdf.
Ethical approval was obtained from the Human Research Ethics Committee of University College Dublin.

| Participants
Participants (n = 1072) were recruited via the platform Prolific (https://www.prolific.co/). Data were collected between 8 June 2021 and 21 June 2021, with participants told the study was focused on media exposure and the COVID-19 pandemic. Participation was offered to participants in six predominantly English-speaking countries (Ireland, United Kingdom, New Zealand, Australia, United States, and Canada) who had not received a COVID-19 vaccine. In line with our preregistration, 220 participants were removed for refusing a postdebrief consent, failing an attention check, and/or reporting having received at least one dose of a COVID-19 vaccine.
In order to avoid revealing the true purpose of the experiment, we did not directly ask participants for their views on vaccination, but instead relied on user information collected by Prolific to categorise participants. Participant data on COVID-19 vaccine opinions was gathered by Prolific, prior to this study, using the question, 'Please describe your attitudes towards the COVID-19 (Coronavirus) vaccines'. Response options were 'I feel positively about the vaccine' (which we labelled as supports COVID vaccines), 'I don't have any strong opinions either way' (labelled as neutral about COVID vaccines), and 'I feel negatively about the vaccine' (labelled as against COVID vaccines). Participant data on general vaccine opinions was gathered using the question, 'Please rate on a scale from 1-7 where 1 is TOTALLY DISAGREE and 7 is TOTALLY AGREE. I believe that scheduled immunizations are safe for children'. For the purposes of the present study, response options 1-3 were labelled anti-vaccine, option 4 was labelled neutral, and options 5-7 were labelled pro-vaccine. As some users revoked their consent for this information to be shared with researchers, these participants were removed from all analyses. The final sample (N = 817) included 277 (33.9%) males, 534 (65.36%) females and 6 (0.73%) other/prefer to self-identify. The mean age was 28.52 years (SD = 8.91).

| Design
This was a between-subjects design, in which participants were randomly assigned to view either a fake anti-vaccination headline or fake pro-vaccination headline, in addition to two true, neutral headlines. A third control condition was included as part of the larger study, but was not relevant to the current study aims and will not be considered here.

| Materials
Ten false headlines were fabricated for use in this study, aiming to mimic the type of misinformation that could be found during the period of data collection. To ensure that each headline was indeed false and novel, extensive online searches were conducted to ensure that the events depicted in the fabricated stories had not actually occurred or been reported. Pilot testing was conducted between March and June 2021, and evaluated each headline's plausibility and likelihood to impact someone's decision to get vaccinated. Since the goal of the larger study was to evaluate effects of misinformation on vaccination intentions, the headlines most likely to affect behaviour were chosen for use in the main study. Specifically, we selected the pro-vaccine headlines which scored the highest, and the anti-vaccine headlines which scored the lowest for behavioural inclination. The headlines were balanced across conditions for plausibility and likelihood to impact behavioural intention to get vaccinated. Importantly, the fabricated headlines all focussed on scientific information and details of the vaccine development, and were not explicitly political in nature. 1 Five pro-vaccine headlines (e.g. 'New study finds risk of lung cancer to be significantly reduced after two shots of COVID-19 vaccine') and five anti-vaccine headlines (e.g. 'The mRNA technology in the COVID-19 vaccine affects cell mutation and decreases your bone density') were selected; see supplementary materials, Data S1 for a complete list of all headlines used. Each fabricated headline was accompanied by a stock image of a vaccine vial.
Five true, neutral headlines were also created, describing actual events relating to the COVID-19 pandemic that were deemed unlikely to influence vaccination decisions (e.g. 'Production for the new Batman movie to be released in 2022 was halted when its star, Robert Pattinson, tested positive for COVID-19'). Similar to the fabricated stories, each headline was accompanied by a relevant image (e.g. a picture of Robert Pattinson dressed as Batman).

| Procedure
The study was presented to participants as aiming 'to investigate reactions to a range of news stories relating to the novel coronavirus outbreak', with neither fake news nor misinformation being mentioned. After consenting to the study, participants reported demographic information (age and gender), and whether they had received at least one dose of the COVID-19 vaccine. Only unvaccinated participants were eligible to continue. They were then randomly assigned to the anti-vaccine or provaccine condition. Participants then viewed three news headlines, in random order. These included one fake headline, randomly selected from the appropriate list of pro-or anti-vaccination headlines, and two randomly selected neutral headlines. Each headline was presented on a separate page, illustrated by an image, and accompanied by the question 'Do you Participants were then debriefed and were re-presented with the headlines accompanied with an explanation of whether each headline was fabricated or true. Following the full debrief, during which participants were informed of the true nature of the study, participants were asked to consent once more to the use of their data for analysis.   whether they reported remembering seeing the novel headlines as the outcome variable. The overall models were not statistically significant (anti-vaccine: χ 2 (2, N = 411) = 2.00, p = .37, Cox-Snell R 2 < .01, Nagelkerke R 2 = .01; pro-vaccine: χ 2 (2, N = 406) = 4.9, p = .085, Cox-Snell R 2 = .01, Nagelkerke R 2 = .02). Individual coefficients are listed in Table 2. The only significant predictor was COVID-19 vaccine opinions for the pro-vaccine headlines (p = .027), such that participants with positive pre-existing views of COVID-19 vaccination were more likely to falsely remember the fabricated pro-vaccination headline.

| RESULTS
The sample in the present analysis was somewhat unbalanced, with the majority of participants reporting pro-vaccination opinions.
In order to increase statistical power, we therefore conducted an additional analysis, not included in the preregistration. The congruence between the headline and the participants' pre-existing COVID-19 vaccine opinion was calculated. Only participants who reported proor anti-vaccine attitudes were included, excluding those who reported neutral views. A chi square analysis was then carried out, comparing the frequency of false memories for attitudinally congruent and incongruent headlines. A significant difference between groups was observed, such that participants were more likely to report false memories for attitudinally congruent headlines (χ 2 (1, 529) = 9.13, p = .003, Cramer's V = 0.13). Congruent headlines were remembered by 15.7% of participants, compared to 7.3% of participants remembering headlines incongruent with their COVID-vaccine attitudes (see Figure 1).

| EXPERIMENT 1 DISCUSSION
The an abortion referendum (Murphy et al., 2019), the feminist movement  and US politics (Frenda et al., 2013 (location and vaccine status) were the same as Experiment 1, with no

| Memories for headlines
Responses were coded as 'remembered'/'did not remember' in the same manner as in Experiment 1. Overall, 33.5% of participants reported remembering the anti-vaccine headlines, while 40.1% remembered the pro-vaccine headlines. The neutral headlines were remembered, on average, by 51.9% of the participants. For both conditions, participants were more likely to remember headlines that aligned with their pre-existing COVID-19 and general vaccine opinions (see Figure 2). Table 3 Table 4. As in Experiment 1, COVID-19 vaccine opinions significantly predicted memories for pro-vaccine headlines (p = .01) but not for anti-vaccine headlines (p = .08).
As with Experiment 1, a follow-up analysis was conducted to maximise power, in which participants were grouped based on whether they had viewed a headline that was congruent or incongruent with their pre-existing opinion on COVID-19 vaccines. Participants who reported feeling neutral on the issue of COVID-19 vaccines were excluded from this analysis. A significant effect of congruency was observed: memories were reported for 44.86% of congruent headlines, compared with just 30.88% of incongruent headlines (χ 2 [1, 431] = 8.96, p = .003, Cramer's V = 0.14).
F I G U R E 2 Percentage of participants reporting memories for true pro-and anti-vaccination headlines, split by pre-existing opinions regarding COVID-19 vaccination. The relative frequency of specific memories (dark colours, bottom) and vague memories (light colours, top) may also be seen.

| GENERAL DISCUSSION
The aim of this study was to investigate the effect of pre-existing vac- considerably more pro-vaccine information in their daily media diets (see Cinelli et al., 2020 for an analysis of the spread of COVID misinformation on social media). As a result participants are likely to have been more familiar with the general tenor of pro-vaccination arguments. It is also possible that they were simply more convinced by pro-vaccine information, regardless of their own position on the issue, and therefore more likely to (falsely) recall having seen it before.
The current research adds to literature suggesting that congruence of a news item with pre-existing beliefs may be a significant contributing factor to false memory formation (Frenda et al., 2013;Murphy et al., 2019Murphy et al., , 2021. This paper builds on recent research which has examined the influence of congruency on simple item-pairing paradigms (e.g. Greve et al., 2019), and provides the first specific evidence of this effect in the context of non-partisan health misinformation. Such findings may be explained via the source monitoring framework; if prior knowledge and opinions guide T A B L E 3 Number and percentage of true pro-and anti-vaccine headlines remembered, categorised by general vaccine and COVID vaccine opinions judgements surrounding memories for past events (e.g. plausibility), source monitoring failures may be more likely to occur for information which is attitudinally congruent (Johnson et al., 1993). The finding may also be explained in terms of fuzzy-trace theory, whereby specific, verbatim features of the news stories are disregarded in favour of the underlying gist (i.e. 'COVID vaccines are good'; 'COVID vaccines are bad'). As the news items in Experiment 2 depicted actual COVID-19 vaccine news events, it appears that attitudinal congruence and corresponding overlap in gist representations may be a significant contributing factor in 'remembering' news items regardless of their basis in truth.
Participants were more likely to report a memory for the true news items presented in Experiment 2 than for the fabricated items in Experiment 1. This may simply be due to the fact that participants had actually encountered these items in the past and were correctly reporting their memories. Given the rate of false memory reports observed in Experiment 1 however, it is likely that at least some of the memory reports for real events in Experiment 2 represent false memories. The current study used a relatively simple paradigm, following similar false-memory studies (e.g. Frenda et al., 2013;Greene & Murphy, 2020;Murphy et al., 2021). The richness of the reported memories for both the accurate and fabricated news items was not assessed, and we cannot comment on whether memories for true and false events were qualitatively distinct. Moreover, we cannot rule out the possibility that some the memory reports in the present study actually reflect participant beliefs that the event occurred, rather than memories of encountering the event in the past (c.f. Wade et al., 2018). Nevertheless, it is clear that participants' willingness to endorse a statement of memory for news events was influenced by their existing beliefs about vaccines, regardless of the truth of the news items.
Multiple exposures to inaccurate information can increase the perceived truthfulness of the information (Fazio et al., 2019;Pennycook et al., 2018), and may have consequences for memory formation too. In order to control for this 'illusory truth effect', novel fake news stories were created for Experiment 1. As all of the events described in Experiment 2 were real news events, it was not possible to control for the influence of previous multiple exposures to the content of the news headlines. It is therefore recommended that future research examine the influence of pre-existing vaccine attitudes on memory reports for vaccine-related news items using paradigms which can control for the influence of previous exposure and which allow for deeper exploration of these memory reports (e.g. longitudinal designs assessing memories over time).
As the current study indicates that individuals are more susceptible to reporting memories or beliefs for both accurate and fabricated vaccine news items which align with their pre-existing vaccine attitudes, the role of attitudinal bias should be considered when developing interventions focused on news literacy and reducing susceptibility to misinformation.

ACKNOWLEDGMENT
Open access funding provided by IReL.

FUNDING INFORMATION
This research was funded by a grant from the Health Research Board of Ireland to CG and GM, grant code COV19-2020-030. The funders had no role in the study design, collection, analysis and interpretation of data, writing of the report or decision to submit the article for publication.